1. Field of the Invention
The present invention relates to a piezoelectric device that includes a thin film made of a piezoelectric single crystal material and a method for manufacturing this piezoelectric device.
2. Description of the Related Art
Many kinds of piezoelectric devices that utilize a piezoelectric thin film, i.e., thin-film piezoelectric devices, are now under development. There are many ways to produce a piezoelectric thin film for the fabrication of thin-film piezoelectric devices. For example, Japanese Unexamined Patent Application Publication No. 2002-534886 proposes a method for manufacturing a piezoelectric device in which a piezoelectric thin film is formed by isolation from a piezoelectric substrate at an ion-implanted layer as the isolation plane.
The following describes this manufacturing method of a piezoelectric device, which includes forming a piezoelectric thin film by isolation from a piezoelectric substrate at an ion-implanted layer as the isolation plane, with reference to FIGS. 1A-1D.
FIGS. 1A-1D are schematic cross-sectional diagrams illustrating the manufacturing steps of a piezoelectric device according to Japanese Unexamined Patent Application Publication No. 2002-534886. The first step is to implant hydrogen ions into a piezoelectric substrate 5 from the front 7 side such that an ion-implanted layer 6 is formed at a predetermined depth d in the piezoelectric substrate 5 as illustrated in FIG. 1A. A coupling material 8 is then deposited on the front 7 of the piezoelectric substrate 5 by sputtering as illustrated in FIG. 1B. The next step is to join the piezoelectric substrate 5 and a supporting substrate 9 as illustrated in FIG. 1C. Finally, the joined body of the piezoelectric substrate 5 and the supporting substrate 9 is heated to initiate isolation at the ion-implanted layer 6 as the isolation plane. As a result, a piezoelectric thin film 5′ is formed on the supporting substrate 9 as illustrated in FIG. 1D.
In the manufacturing method according to Japanese Unexamined Patent Application Publication No. 2002-534886, however, ion implantation (see FIG. 1A) causes the piezoelectric substrate 5 warped with the ion-implanted layer 6 side of the piezoelectric substrate 5 to be curved outwards as illustrated in FIG. 2A. This type of warpage occurs because the implanted ionized element causes the crystal lattice spacing of the piezoelectric material to extend in the ion-implanted portion of the piezoelectric substrate 5.
When the piezoelectric substrate 5 is in such a warped shape, bonding it to a supporting substrate 9 (see FIG. 1C) results in the piezoelectric material existing on the surface 7 on the ion-implanted layer 6 side adhering to the supporting substrate 9 with its crystal lattice spacing longer than that before ion implantation. Isolation at the ion-implanted layer after bonding to the supporting substrate 9 therefore causes the piezoelectric thin film 5′ to produce a compressive stress that compresses the back 4 of the supporting substrate 9, which is opposite to the surface on the piezoelectric thin film 5′ side, making the supporting substrate 9 after isolation warped with the piezoelectric thin film 5′ side curved outwards as illustrated in FIG. 2B.
This means that piezoelectric thin film devices produced by the manufacturing method according to Japanese Unexamined Patent Application Publication No. 2002-534886 are disadvantageous because the compressive stress exerted on the isolation plane during this isolation operation leads to a degraded surface roughness of the piezoelectric thin film 5′. Worse yet, this compressive stress is localized in the isolation plane, and the supporting substrate 9 after isolation is likely to crack.